I use a Ram mount for my navigation Ipaq. It's a powered model with a 9-pin d-sub for serial. It's built really well and they are very confgurable. You could get it set up for a bike with a mount for the Ipaq and another mount that connects to it for the GPS.

The only issue I have is that the power supply is noisy and drowns out all but the strongest FM radio stations. This is why that mount isn't recommened for use in sailplanes.